Passage 3

Dinosaur Extinction

① Dinosaurs, one of a group of extinct reptiles that lived from about 210 million to about 61 million years ago, once reigned as the dominant animals on land for more than 140 million years. Yet at the end of the Cretaceous Period, nearly all members of this species vanished off the face of the earth, a mass extinction that still lingers in the popular imagination. What caused this mass extinction? Scientists have yet to find an answer that can explain why they died out while most mammals, turtles, crocodiles, salamanders, and frogs survived.

② One theory sees the extinction of dinosaurs as the result of environmental changes linked to the gradual withdrawal of shallow seas from the continents at the end of the dinosaurian era.【1】Proponents of this theory postulate that dinosaurs dwindled in number and variety over several millions of years.

③ Most scientists, however, tend to huddle around one of two hypotheses that may explain the Cretaceous extinction: an extraterrestrial impact, such as one from an asteroid or comet, or a massive bout of volcanism. Either scenario would have choked the skies with debris that starved the Earth of the sun's energy, throwing a wrench in photosynthesis and sending destruction up and down the food chain. Once the dust settled, greenhouse gases locked in the atmosphere would have caused the temperature to soar, a swift climate swing to topple much of the life that survived the prolonged darkness.

④ The extraterrestrial impact theory stems from the discovery that a layer of rock dated precisely on the extinction event is rich in the metal iridium. This layer is found all over the world, on land and in the oceans. Normally iridium is rare on Earth but it's found in meteorites at the same concentration as in this layer. This led scientists to postulate that the iridium was scattered worldwide when a comet or asteroid struck somewhere on Earth and then vaporized. A 110-mile-wide (180-kilometer-wide) crater carved out of Mexico's Yucatán Peninsula, called Chicxulub, has since been found and dated to 65 million years ago. Many scientists believe the fallout from the impact killed the dinosaurs.

⑤ But the Earth's core is also rich in iridium, and the core is the source of magma. Some scientists say magma spewed out in vast, floodlike flows that piled up more than 1.5 miles (2.4 kilometers) thick over 1 million square miles (2.6 million square kilometers) in India. This bout of volcanism has also been dated to about 61 million years ago and would have spread the iridium around the world, along with sunlight-blocking dust and soot and greenhouse gases.

⑥ Both the Asteroid and Volcanoes hypotheses have merit. Some scientists think both may have contributed to the extinction. Regardless of what caused the extinction, it marked the end of the Tyrannosaurus rex's reign of terror and opened the door for mammals to rapidly diversify and evolve into newly opened niches.

——2012年 4月 20日北美机经

Why does the author mention the crater Chicxulub in Paragraph 4?

A. To introduce the Volcanoes hypotheses.

B. To prove that a comet or asteroid once struck somewhere on Earth.

C. To give concrete evidence to the extraterrestrial impact theory.

D. To explain where the iridium come from.

词汇练习：

阅读下列句子，用所给单词（或词组）的正确形式填空：

comet

postulate

debris

extinction

crater

photosynthesis

withdrawal

proponent

reptile

asteroid

dominant

prolong

1. It swims farther into the cold of the northern and southern oceans than any other sea turtle, and it deals with the chilly waters in a way unique among ______.（TPO-11：A Warm-Blooded Turtle）

2. Although the ecosystem concept was very popular in the 1150s and 1160s, it is no longer the ______ paradigm.（TPO-11：Succession, Climax, and Ecosystems）

3. Why did the near ______ and migrations occur?（TPO-7：The Geologic History of the Mediterranean）

4. It is estimated that at current ______ rates, much of the aquifer will run dry within 40 years.（TPO-1：Depletion of the Ogallala Aquifer）

5. ______ point to features such as the terraced“beaches” shown in one image, which could conceivably have been left behind as a lake or ocean evaporated and the shoreline receded.（TPO-8：Running Water on Mars）

6. Rayleigh and Ramsay ______ the existence of a new group of elements, and in 1818 other members of the series (neon, krypton, and xenon) were isolated.（TPO-16：Development of the Periodic Table）

7. In view of these facts, scientists hypothesized that a single large ______, about 10 to 15 kilometers across, collided with Earth, and the resulting fallout created the boundary clay. (TPO-8：Extinction of the Dinosaurs）

8. The Sun is the hub of a huge rotating system consisting of nine planets, their satellites, and numerous small bodies, including asteroids, ______, and meteoroids.（TPO-16：Planets in Our Solar System）

9. These springs are the sources of streams and rivers, which cut through the rocks and carry away ______ from the mountains to the lowlands.（OG：Geology and Landscape）

10. The algae capture the sun's energy by ______ and store it in organic molecules.（TPO-1：The Arrival of Plant Life in Hawaii）

11. But the inhabitants of Tikal and other cities dependent on reservoirs for drinking water would still have been in deep trouble if 18 months passed without rain in a ______ drought. （TPO-14：Maya Water Problems）

12. Even diluted by the terrestrial material excavated from the ______, this component of meteorites is easily identified.（Official Model Exam：Meteorite Impact and Dinosaur Extinction）

参考答案：

1. reptiles

2. dominant

3. extinction

4. withdrawal

5. Proponents

6. postulated

7. asteroid

8. comets

9. debris

10. photosynthesis

11. prolonged

12. crater

【1】与今日百句译（Sentence 3）相同的句型。